The present invention generally relates to source material feeders used in industrial crystal growth processes. More particularly, the present invention resides in a source material feeder for such processes which converts batch process devices and systems into semi-continuous or continuous processes using granular or irregular shaped feed particles and controlled feeding into a large number of different crystal growth systems.
Synthetic crystal growth has been industrialized for decades, with little industry-wide process optimization. Innovation is usually confined within individual organizations that tend to keep new ideas secret. As a result, several unique and complex growing techniques have evolved, with growth equipment designed specifically for each manufacturer's process. Manufacturing challenges also vary among the different methods used. For instance, batch crystal growers are limited by the initial capacity of the melt crucible and growers who use shaped crystal growth methods have little or no options when selecting silicon source materials. Even though there are significant differences in method, the factors that drive operating costs are similar: raw materials such as polysilicon; consumable items like quartz crucibles; and electricity are major cost contributors.
Of all the crystalline materials manufactured commercially, silicon is in the highest demand. Most industrially grown silicon and other materials are still grown using inefficient batch processes. There are basically three types of specialized feeders presently available: (1) internal feed hoppers/rods that hang from seed cables; (2) gravity-fed external feeders; and (3) metering feeders used for continuous feeding of pellet/granular silicon. Some feeders are compatible only with specific types of crystal growth furnaces, and cannot be retrofit into other systems. Other designs are intended to feed only specific types of source material, such as a pelletized material. None can perform metered feeding of alternative source materials.
Most silicon is grown using the Czochralski (CZ) batch process. In this process, a silicon ingot is made by melting source materials in a crucible, dipping a seed crystal into the melt, and withdrawing the seed in such a manner as to achieve a single crystal of a specified diameter. Operators must then power-down and disassemble the furnace to reload it with a fresh crucible and source material after each ingot pull.
However, the CZ batch process has many limitations. The charge size is limited to approximately 60% of the actual crucible capacity as the unmelted raw material occupies a greater volume than the melted material. Throughput is hindered by non-productive set-up, heat-up, and cool-down cycles. This also increases energy costs significantly. Approximately 7-8% of the charge is unusable melt residue which remains attached to the crucible. Thus, the quartz crucibles, which are rather expensive (currently $500-$600 each), cannot be reused. In fact, the quartz crucible will actually crack due to the thermal cycling. Other furnace hot-zone components also break down due to thermal cycling and excessive handling. Moreover, advanced hot-zone processes that permit faster pull speeds and higher quality crystals cannot be used without further reducing crucible capacity.
To overcome these limitations, some have tried adding additional source materials. One technique uses an internal hopper or polysilicon rod that hangs from the seed cable to add materials after meltdown or in-between crystal pulls. However, efficiency gains using this method are offset by extra cycle time needed because the grown ingot has to be cooled and removed before the hopper is installed.
Others use an external feed hopper to feed pellet/granular silicon source material into the furnace without waiting for ingot removal. U.S. Pat. No. 5,462,010 to Takano et al. is directed to an apparatus which continuously supplies granular polycrystal silicon to a crucible of a semi-conductor single crystal pulling apparatus. Takano et al. disclose that the apparatus includes a main tank holding a large supply of the granular silicon source material. The granular material is fed into a sub-hopper having a supply controller in the form of rotary valves in a tube which exits into a smaller main hopper. Due to the granular/pellet nature of the material, the rotary valves can be selectively opened to permit the material to flow into the main hopper. The weight of the main hopper is monitored and the rotary valves opened to supply additional material as the crystal batch process proceeds and additional material is required.
This technique has proven very successful for a few companies, but requires source material manufactured by a proprietary fluidized bed process. This material is commercially available from only one company and expensive compared to other sources of raw material. Such granularized material has also been found to have undesirable characteristics resulting from the fluidized bed process. Thus, many industrial crystal growers prefer not to use the granular material. In addition to supply concerns, some existing feed hoppers are not designed to be refilled during a furnace run, and most are gravity-fed with little or no control of flow rate.
Batch casting techniques such as the Heat Exchange Method (HEM) are also used to grow single or polycrystalline silicon ingots. To cast a silicon ingot, source materials are loaded into a crucible and melted. The furnace temperature is lowered in a controlled way to directionally solidify the silicon. When growth is complete, the furnace is shut down. The limiting factor in casting ingots is crucible capacity. The unmelted raw material occupies a greater volume than the melted material.
Several commercial growing processes grow crystals to shape, usually in the form of ribbon or sheet. The Edge-Defined, Film-fed Growth (EFG) method, the String-Ribbon process and the Dendritic-Web technique all utilize continuous melt replenishment (CMR) during crystal growth. This means that silicon source materials are replaced, gram for gram, as the crystals are grown. CZ crystal growers can also apply this technology in order to gain better control of the electrical properties of the grown ingot. However, feeders currently available to perform continuous melt replenishment only accommodate the expensive pellet/granular silicon source materials. Therefore, users are locked into a single source supply, and CZ growers are reluctant to adopt the technology.
Accordingly, there is a continuing need for a feeder of source material for industrial crystal growth processes and systems which can convert the batch process into a semi-continuous or continuous process, and/or increasing the initial crucible volume, resulting in increased manufacturing efficiency. What is also needed is such a feeder which can be retrofit into a variety of existing systems. Moreover, such a feeder should be capable of accommodating large, irregular shaped feed particles and provide controlled feeding into the crystal growth system. The present invention fulfills these needs and provides other related advantages.